On my NC20 I had twofactor_totp installed. To verify the reporter’s issue I deactivated and uninstalled it. I confirm that twofactor_totp is not shown when trying to search for it or install it from a category in /settings/apps. OCC doesn’t help either
$ php occ app:install twofactor_totp
Error: Could not download app twofactor_totp
I suspect compatibility markings: 5.0.0 is the latest version marked compatible with NC20 – according to the app store. It should be selected. Instead, 6.0.0 (marked compatible for NC21 only) is selected but filtered out due to its incompatibility.
This was reported as bug ending up in this PR but the issue persists in 20.0.6
Theoretically it could also be due to app signatures or a stale download link.